In Multiple Input Multiple Output (MIMO) Orthogonal Frequency Division Multiplexing Access (OFDMA) cellular systems, closed-loop MIMO precoding is defined to enable high throughput downlink transmissions to fixed and nomadic user terminals. Feedback information is obtained by a transmitting device via a report of a precoding matrix index (PMI) from a receiving device. This PMI is used to represent a matrix in a codebook (a codebook is a set of matrices), and the PMI is used by the transmitting device to derive a downlink precoder. While the concepts of “transmitting device” and “receiving device” are relevant to each other, in the scenarios involved in this application, the transmitting device may be in a form of a network entity, such as a base station, an evolved NodeB (eNB) or an access point. The receiving device may be in a form of a user equipment (UE), such as a mobile device, a cellular telephone, a wireless enabled personal digital assistant (PDA), a wireless enabled laptop computer, or other such devices. However, it is noted that the present application is not limited by these scenarios.
Simple codebook structures are used in the first generation of MIMO OFDMA systems, such that the PMI can be represented in a few bits. Small codebooks can be used to achieve a coarse quantization of a spatial channel. For example, a 4-bit codebook is defined and a 4-bit PMI can be reported via a capacity-limited feedback channel called Physical uplink control channel (PUCCH). Precoding matrices are defined for each possible transmission rank, where the transmission rank determines the size of the precoding matrix. The overall codebook is a set of matrices for each rank. The feedback report thus includes a rank indication (RI) and a PMI in the codebook subset of the given rank. Since the rank of the propagation channel varies slowly in comparison with fast fading over which the PMI is adapted, the rank indication is reported with a longer period than the PMI.
Since the RI remains valid during several reports of the PMI, it can be encoded with a better error protection code in order to ensure that consecutive PMI reports are not invalidated by one erroneous RI report. In general, this is naturally ensured by the fact that the rank can take values in a limited range, such as {1, 2, 3, 4} in LTE (Long Term Evolution) system Rel-8, due to the limitation to a maximum of 4 antennas at the transmitting device and 4 antennas at the receiving device. Thus the RI can be represented by 2 bits. Since the PMI is in general reported along with a channel quality indication (CQI) that represents the channel quality, assuming that the transmitting device precodes data with the reported PMI, the total size of the feedback message that contains the PMI+CQI is larger than the 4 bits used to represent just the PMI. For example, in LTE Rel-8, the message size will be 8 to 11 bits with one to two CQIs, respectively. The PUCCH is transmitted in a fixed-size time-frequency resource with a fixed modulation. The 2 bits of RI that are reported individually are better protected than the 8 or 11 bits of PMI and CQI that are jointly reported, since the 2 bits of RI enjoy a lower error-correction encoding rate.